1. Field of the Invention
This invention relates to auto-alignment of mirrors in a laser resonator, and more particularly to auto-alignment of a high energy laser by a small laser beam external to the gain medium.
2. Description of the Related Art
On a conventional ground based high energy laser (HEL), such as shown in FIGS. 1 and 2, the output power extraction optics are configured in a standard negative branch confocal unstable resonator of fairly high magnification. These resonators yield a HEL beam that is collimated and well directed, and since the scraper mirror 3 is self-imaged in the other two resonator mirrors 1 and 2, the output beam is free from diffraction. Typically, in order to keep beam misalignment and jitter to a low value, the, resonator optics are mounted to a massive (and vibrationless) optical bench structure which is usually constructed of concrete. FIG. 1 shows the simplest configuration where as FIG. 2 shows the same resonator with a polarization control flat mirror 4 included in the feedback leg of the resonator. (In FIG. 1 the HEL is delivered in an upwards direction whereas in FIG. 2 it is delivered out of the plane of the paper.) When such resonators are used in an airborne configuration the optical bench for the airborne system must be light and therefore relatively weak, the optics will therefore vibrate and move relative to each other which will destroy the laser beam quality and cause the beam to jitter in a dangerous manner within the aircraft. Therefore an auto-alignment system wherein the resonator optical components remain constantly self-align, must be provided. Further since the optical gain region is not always accessible, the auto-alignment system must access the resonator mirrors from outside the gain medium region 14. In addition, because the HEL output may drift relative to the aircraft, a surrogate beam that continuously represents the HEL pointing direction needs to be provided.
The invention provides a small laser beam that emanates from an attachment on the feedback mirror (FBM) 2 of the resonator, goes through an extra-cavity beam tube 5 to sense the position of each of the resonator mirrors and then returns to its point of origin. Upon return the small beam contains information about mirror misalignment errors and when it is sensed it provides error signals to drive one mirror so the resonator returns to alignment.
Before returning to the point of origin, a part of this beam is split off at the scraper mirror and is directed towards the beam control system as a surrogate beam that represents the pointing of the HEL.
It is an object of the invention to save considerable weight in the airborne HEL system by drastically reducing the weight of the optical bench.
It is also an object of the invention to provide a surrogate beam which continuously represents the output of the HEL.
It is a further object of the invention to mitigate beam jitter by correcting for relative movement of the optics in a resonator.
It is still a further object of the invention to improve the beam quality of a high energy laser beam emanating from a laser resonator.